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An employee packages a customer order ahead of shipping at one of Amazon.com Inc.'s fulfillment centers in Rugeley, U.K., on Monday, Dec. 2, 2013. Online retailers in the U.K. are anticipating their busiest day as shoppers flush with end-of-month pay-checks seek Christmas deals on the Web.

Cyber Monday is still on top.

Retailers from Wal-Mart Stores to Amazon started rolling out "Cyber" deals at the beginning of November, and kept them going on Thanksgiving and Black Friday. That led some to wonder if earlier sales would put a dent in Cyber Monday sales. The date has been the biggest online shopping day of the year since 2010.

But shoppers delivered. In fact, shoppers bought online at the heaviest rate ever Monday, according to research firm comScore Inc., which tracks online sales.

The group said Tuesday e-commerce spending rose 18 percent from last year's Cyber Monday to $1.74 billion, making Monday the top online spending day since comScore began tracking the data in 2001. The figure does not include purchases from mobile devices.

"I always wait for the deals on Cyber Monday," said Stephanie Appiah, 25, a student who picked up a Google Chromecast video streamer with free shipping on Monday. "It's better than Black Friday because you don't have to deal with other people."

The strong online performance was in contrast to overall spending. Over the four days beginning on Thanksgiving, spending fell an estimated 2.9 percent to $57.4 billion, according to the trade group the National Retail Federation. Overall, the NRF expects holiday spending to rise 2.9 percent to $602.1 billion.

"Any notion that Cyber Monday is declining in importance appears to be completely unfounded," comScore Chairman Gian Fulgoni said in a statement Tuesday. "While it's true that many retailers are bleeding their Cyber Monday promotions into the weekend before and the days afterward, Cyber Monday itself continues to be the most important day of the online holiday shopping season."

However, he did say that early promotions had some consumers buying more items earlier in the weekend, suggesting that Cyber Monday could have even been stronger were it not for the emergence of this trend.

Consumer electronics and video game consoles and accessories were among the biggest sellers of the day. Home and garden products, clothing and accessories, as well as sports and fitness products also performed well.

ComScore tracks U.S. online sales based on observed behavior of a representative U.S. consumer panel of 1 million Web users.

One big online shopping trend so far this year is shoppers researching and buying on mobile devices like smartphones and tablets, said Forrester Research analyst Sucharita Mulpuru.

"There was an enormous lift in the number of people who use mobile devices, and it's been trending that way for the last couple of weeks," she said. Forrester forecasts $78.7 billion in U.S. online sales this holiday season, a 15 percent increase over 2012.

Meanwhile, IBM Benchmark reported on Tuesday that Cyber Monday sales rose 20.6 percent. IBM Benchmark takes sales results from over 500 online retailers and analyzes the data to estimate total online spending. Mobile sales, including smartphones and tablets, made up 17 percent of total online sales, an increase of 55.4 percent compared with last year.

Department stores were the strongest performers, with sales up 70 percent. The group does not give dollar amounts. Over the five-day shopping period beginning on Thanksgiving, sales rose 16.5 percent compared with 2012.

Jay Henderson, strategy director of IBM Smarter Commerce, said the fact that there were strong online sales on Thanksgiving, Black Friday and Cyber Monday bodes well for the rest of the season.

"We should see continued growth straight through the holiday season," he said. "Consumers seem to be online and spending in force."

The name Cyber Monday was coined in 2005 by NRF's online arm, called Shop.org, to encourage people to shop online. After retailers revved up deals for the day, it became the busiest online shopping day in 2010.

The name was also a nod to online shopping being done at work where faster connections made it easier to browse.

ComScore said Tuesday that even with high-speed connections being the norm these days, nearly half of consumers are still shopping online at work on Cyber Monday.